Monocrystalline material is used in the manufacture of a wide variety of products, such as electrical circuitry, optical systems, and other microminiature devices. There are a number of methods that are utilized in the growth of a monocrystalline structure from a melt. A synopsis of some of the more significant methods is given below.
The Czochralski method involves the melting of a polycrystalline charge in a crucible by radio frequency induction or resistance heating. A monocrystalline seed is then lowered into the melt while being rotated in a clockwise direction. The crucible and its charge are rotated in a counterclockwise direction. The seed crystal is withdrawn at a slow rate from the melt until the desired diameter of the pulled monocrystalline structure is obtained. The pull speed is increased to maintain the desired diameter of the pull. This procedure continues as long as there is melt remaining in the crucible. One problem encountered with the Czochralski method is that of controlling the cross-sectional area of the crystal. While a circular cross-section is produced, the diameter of the crystal has a tendency to vary widely as the growth proceeds. An additional disadvantage is that the monocrystalline structure pulled from the charge may be contaminated by the material of the crucible.
The Stepanov method (See A. V. Stepanov, Bull. Acad. Sci. USSR, vol. 33 (1969), p. 1775) is a modification of the conventional Czochralski method. By the Stepanov method, a die member is mounted at a fixed position within the crucible such that the upper edges of the die are above the surface of the melt and the bottom of the die is well below the surface of the melt. The key to Stephanov's technique is shaping a melt column, and the melt column shape is used to control the crystal shape. See H. E. LaBelle, Jr., J. Crystal Growth, Vol. 50, 1980, p. 8. A difficulty with the Stepanov method is that constant control of the melt level in the crucible is required, as the level of the melt and hence the shape of the melt column will vary upon formation of the crystal.
In the edge-defined, film-fed growth (EFG) technique, the shape of the crystalline body is determined by the external or edge configuration of the end surface of a forming member or die. An advantage of the process is that bodies of selected shapes such as round tubes or flat ribbons can be produced. The process involves growth on a seed from a liquid film of feed material sandwiched between the growing body and the end surface of the die, with the liquid in the film being continuously replenished from a suitable melt reservoir via one or more capillaries in the die member. By appropriately controlling the pulling speed of the growing body and the temperature of the liquid film, the film can be made to spread (under the influence of the surface tension at its periphery) across the full expanse of the end surface of the die until it reaches the perimeter or perimeters thereof formed by intersection of that surface with the side surface or surfaces of the die. The angle of intersection of the aforesaid surfaces of the die is such relative to the contact angle of the liquid film that the liquid's surface tension will prevent it from overrunning the edge or edges of the die's end surface. The growing body grows to the shape of the film which conforms to the edge configuration of the die's end surface.
The Bridgman-Stockbarger method utilizes an elongated container of material which is melted in a high temperature furnace, after which the container is lowered into a cooler, lower temperature furnace, which allows the material to slowly resolidify as a single crystal. The molten material from which the crystal is grown is completely enclosed during the process, and as a result, strains occur in the material which induce defects when the molten material solidifies.
Float zone refining is another method used to convert polycrystalline material to a high quality monocrystalline rod and, simultaneously, to remove unwanted impurities from the material. In the float zone technique a narrow molten zone is caused to move slowly along the length of a vertically disposed rod of polycrystalline material. As the molten zone moves, the material immediately behind the zone resolidifies as monocrystalline material. The monocrystalline growth is initially nucleated by a single crystal seed and then continues in a self-seeding manner. Impurities in the material tend to congregate in front of the molten zone so that as the as the molten zone moves, the zone also removes impurities with it, leaving the material behind the zone in a purer state.
In the float zone process with a contactless heater, the molten zone is caused to traverse the length of the polycrystalline rod by moving the rod vertically downward past a stationary heating means such as a radio frequency induction coil that surrounds a material in the contactless manner. In an alternate embodiment of the float zone refining process with a contactless heater, the rod is stationary and the heater moves vertically across the length of the rod. In addition to the translational motion, a rotational motion may also be imparted to improve crystal perfection and uniformity. The float zone process with a contactless heater, while producing a clean monocrystalline result, is very unstable in that the melt zone tends to collapse. Other difficulties in the float zone process with a contactless heater include the presence of a strong thermocapillary flow in the melt that results in banding and the fact that the melt/crystal interface is convex (rather than flat) toward the melt, resulting in solute segregation and dislocations.
Float zone refining has evolved to incorporate a horizontal resistance sheet heater immersed in the melt zone instead of surrounding the material in a contactless manner, the melt flowing through a hole or holes in the heater. The float zone immersed heater process allows more precise control over the growth front, i.e., the melt/crystal interfaces. A flat growth front can be readily maintained in the immersed heater process, mainly because the heater is immediately adjacent to the growth front. Besides control over the growth front, the immersed-heater process has several additional advantages over other crystal growth processes. First, unlike the Bridgman-Stockbarger process, crystal "straining" or "sticking" caused by growing the crystal inside a crucible, cannot occur. Second, unlike in the floating-zone process without an immersed heater, the melt zone is very stable in the immersed-heater process. This is mainly because the melt zone is much shorter in the latter (usually around 1-2 mm) and, therefore, has a much smaller tendency to collapse. Third, growth initiation with a seed crystal is particularly easy with the immersed-heater process. The heater is initially in direct contact with the seed crystal. Consequently, the extent of melting in the seed crystal can be accurately controlled to insure a proper growth initiation.
The immersed-heater process was first used by Gasson (See D. B. Gasson, J. Sci. Instrum., vol. 42 (1965), pp. 114-15) in 1965, for the growth of neodymium-doped single crystals of calcium tungstate (CaW0.sub.4, melting point 1620.degree. C.) for laser rods. In the preparation of such crystals by the Czochralski process, Gasson observed that the shape of the growth front was not only rather convex (toward the melt) but also quite difficult to control. The resultant crystals exhibited nonuniform optical transmission and this optical nonuniformity varied from crystal to crystal. In order to gain a precise control over the shape of the growth front and hence improve the quality and consistency of the crystals, an iridium immersed heater was employed. The heater was 1 mm thick, 10 mm wide, and 50 mm long, with two 1 mm-diameter holes near its center. The crystals, which were rotated at 60 rpm during growth, were 6 mm in diameter. They were reported to show better optical quality and consistency than the Czochralski pulled crystals.
The immersed-heater process was subsequently applied to the growth of the following crystals: 10 mm diameter CaCO.sub.3 (See J. J. Brissot and C. Belin, J. Cryst. Growth, Vol. 8 (1971), pp. 213-15), 25 mm diameter CaCO.sub.3 (See C. Belin, J. Cryst. Growth, Vol. 34 (1976), pp. 341-44), 8 mm diameter BaTiO.sub.3 (See C. E. Turner, N. H. Mason and A. W. Morris, J. Cryst. Growth, vol. 56 (1982), pp. 137-40), and 8 mm diameter Ba.sub.0.65 Sr.sub.0.35 TiO.sub.3 (See R. M. Hensen and A. J. Pointon, J. Cryst. Growth, vol. 26 (1974), pp. 174-76). A platinum sheet heater was used in all these cases. The use of the immersed-heater process to grow single crystals of hexagonal selenium has also been considered. See P. R. Swinehart, J. Cryst. Growth, vol. 26 (1974), pp. 317-18. More recently, 20 mm diameter crystals of LiNbO.sub.3 have been grown using a platinum sheet heater (U.S. Pat. No. 4,752,451 issued to Aubert et al.) In the latter case, the heater was held stationary, and the feed rod and the growing crystal were moved and rotated. The moving and rotational speeds of the feed rod were different from those of the crystal. The diameter of crystals grown by the immersed-heater process often varies significantly along the crystal and the crystal surface is often rough. The main disadvantage of the immersed-heater process is that applications are limited by the requirement of chemical capability between the melt and the immersed heater.